In recent years, there has been developed a series of microprocessors of the type used to process digital data in a manner similar to digital computers. These microprocessors are integrated circuit chips which generally include a series of output address lines, four or eight bi-directional data lines, and a series of command lines, such as READ, WRITE, WAIT, READY, HOLD and synchronization. With the advent of this type of miniaturized device, digital processing of information for a variety of applications has become possible and relatively inexpensive. Most development work has been directed toward the concept of utilizing the relatively limited terminals of a microprocessor to process the digital data in a series of machine cycles divided into a series of micro cycles. In accordance with the application incorporated by reference herein, a microprocessor has been incorporated into a programmable controller in a manner that recognizes the limitations of the microprocessor and provides substantial external hardwire modules to employ the limited functional characteristics of the microprocessor in a total system having the capabilities of inputting and outputting either bit or byte digital data. Consequently, the programmable controller using a microprocessor can control various machines, processes, and other sequential manipulations. This type of system, to be universal in nature, requires the use of a conversion circuit for converting input analog signals into digital information available on the data lines of the microprocessor forming the central part or central processing unit of the programmable controller. Various circuits are known for converting analog to digital information. These circuits include a device for inputting an analog voltage and converting the magnitude of the voltage into digital data on output terminals to be used by data lines of the system, such as a digital computer. These available converting devices usually employ a multiple bit output data representative of the magnitude of the input analog signal. Generally, the digital data is the output of a binary counter having several stages, such as twelve. Thus, a full scale conversion of an input voltage signal by such a converting circuit requires ten output terminals to read between 0 and 999. This would give a full scale conversion of the input signal between 0% and 99.9%. One additional terminal is generally used to show that the counter has started to count. Consequently, eleven or twelve output data terminals are employed at the output of an analog conversion circuit of the general nature used in converting analog input voltage signals into output digital full scale readings. Such a conversion circuit is well adapted for digital computers having a greater number of data lines; however, when using a programmable controller employing a microprocessor, there are insufficient data lines for accepting full scale digital output data for the input analog signal. Thus, if ten output terminals are required for a full scale reading, a microprocessor having only four data terminals can not accept the digital information in its output form from a standard type of conversion circuit. The same is true of the higher capacity microprocessor having eight data lines. For that reason, efforts have been made to provide a standard conversion circuit for use in a microprocessor having a limited number of data lines.
In systems to be controlled by programmable controller, it is often necessary to input a large number of analog signals, such as thermocouple voltages, speed analog signals and position analog signals. Consequently, any analog system for general purpose use with a programmable controller must have an analog input circuit for accepting a large number of analog signals and converting them to digital data for use on the bidirectional data lines of the microprocessor. This has presented a substantial amount of difficulty in adapting a microprocessor for use in a programmable controller system. As a first proposal for accomplishing this feat, it was suggested that the number of digital converting circuits be increased as the input analog signal increased. This substantially added to the size and cost of a programmable controller, the advantage of which is the reduction in size and a low capital investment. Thus, such an expansion arrangement was counterproductive to the basic concept of using a microprocessor in a programmable controller. The present invention relates to an analog system for inputting analog signals into a programmable controller using a microprocessor, with its inherent limitations, which system allows a large number of input analog signals to be processed using only a single analog to digital conversion circuit.